Process for separating mixtures of aromatic C8 hydrocarbons

The present invention relates to a process for separating the constituents of a mixture of aromatic C 8 hydrocarbons, which substantially comprises a preliminary separation of the mixture into two streams, the first one of which contains paraxylene and ethylbenzene, and the other one is constituted by meta- and ortho-xylene, and two subsequent distinct operations of rectification during the course of which ethylbenzene is recovered as the overhead fraction, and ortho-xylene is separated as the bottom fraction.

As is well-known, the production of aromatic hydrocarbons is greatly interested for the whole chemical industry. For that purpose, rather simple processes exist, in particular, for the purpose of separating aromatic from non-aromatic species; the resulting mixture of aromatic species is easily separated by distillation into C 6 aromatics (benzene) with a suitable purity for any possible use, and into C 7 aromatics (toluene), also with a high purity level.

The situation is completely different as regards C 8 aromatics, the mixtures of which not always easily be separated, because they are constituted by mixtures of isomers with very close boiling points. The most interesting C 8 mixture from the industrial viewpoint is that mixture which is constituted by ethylbenzene, o-xylene, p-xylene and m-xylene: as such, said mixture is used as a solvent or in gasolines, whilst much wider would be the range of possible uses of each individual isomer, if each could be separated with high purity level. So, a demand exists for high-purity para-xylene for preparing terephthalic acid and dimethyl terephthalate, intermediates for man-made fibre synthesis. Ethylbenzene, in turn, is an interesting intermediate in styrene preparation by dehydrogenation.

Usually, the processes used heretofore in order to separate the above said isomers have been mostly based on suitable combinations of superfractionation techniques and of low-temperature crystallation processes: all the above having high costs and limited yields.

According to alternative routes, the para-isomer is separated by adsorption, or by using molecular sieves: the various isomers can then be isomerized and the produced para-xylene can be separated again according to the above mentioned technique.

The ortho- and ethylbenzene isomers can also be separated by distillation.

However, the above mentioned para-isomer separation process by crystallization does not make it possible for said isomers to be totally recovered; the presence of the product in the mother liquors sent to the isomerization reduces the isomerization yield per pass, and increases the need for recycles.

Furthermore, the presence of ethylbenzene causes even more serious problems, because this produce tends to accumulate in the isomerization step, causing still more serious recycling problems. If possible, then, its conversion into dimethyl benzene always takes place with low yields and serious operation problems.

From U.S. Pat. No. 3,698,157, a process is known as well for recovering para-xylene from a mixture of C 8 aromatic by contacting said mixture with a suitably modified zeolite. Also the formation is envisaged of two fractions constituted by:

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meta Pboil:

139.12° C.

| para Pboil:

138.37° C.

ortho Pboil:

144.41° C.

| ethyl-b Pboil:

136.19° C.

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Such fractions can then be separated by distillation. The meta/ortho mixture can yield the meta-isomer as the overhead fraction and the ortho-isomer as the bottom fraction, with reflux ratios of the order of 10-20, and with a number of trays comprises within the range of from 150 to 200.

The para/ethyl-b mixture can also be separated by distillation, under burdensome conditions (however, obviously, not as burdensome as they would be if the whole stream were submitted to distillation).

FIG. 1 shows the scheme used in Example 1.

FIG. 2 shows the scheme used in Example 2.

FIG. 3 shows the scheme used in Example 3.

The present Applicants have found now that the four isomers of the mixture of C 8 aromatics can be separated in a simple and cheap way by adopting a processing cycle which substantially comprises a preliminary separation of said mixture into two streams, and subsequent rectification operations.

The composition of a typical mixture of the interesting isomer can be represented as follows:

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ethylbenzene

20

para-xylene

20

meta-xylene

40

ortho-xylene

20

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In the first step of the cycle followed in the process according to the present invention, the mixture is sent to a separation zone containing a suitable agent, in which zone para-xylene and ethylbenzene are separated from the other binary mixture consisting of ortho- and meta-xylene.

This separation can be carried out by using well known means for those skilled in the art, such as, e.g., those disclosed in the above mentioned U.S. Pat. No. 3,698,157, or in U.S. Pat. No. 3,656,278.

The ethylbenzene and para-xylene-containing stream is then sent to a rectification column operating with a reflux ratio within the range of from 50 to 80, and containing from 300 to 400 trays.

Ethylbenzene within specifications and paraxylene are obtained as the overhead and bottom fractions, respectively.

The latter may also contain 1 to 10% by weight of other isomers: in such a case, a further purification will be carried out by crystallization, and the mother liquors will be recycled.

The parallel ortho-xylene and meta-xylene mixture is sent, in turn, to a rectification column in which said isomers are separated at respective purity levels higher than 99%: ortho-xylene as the bottom fraction, and meta-xylene as the overhead fraction, both of the them being obtained with practically quantitative yields relative to the feedstock.

In the ortho-xylene fraction impurities constituted by C 9 species can still be present, which will simply be distilled off. Should the production of meta- and ortho-isomers not be interesting, the mixture can suitably be sent to an isomerization step: in such a case, the operating cycle according to the present invention will produce only ethylbenzene and para-xylene with purity levels within specification, because the effluent stream form the isomerization zone will also be fed to a separation step on-line with the initial separation step.

Also the isomerization, if present, is carried out according to well-known techniques for those skilled in the art.

The rectification column for ortho-xylene/metaxylene mixture operates, according to the cycle according to the present invention, with a reflux ratio within the range of from 10 to 20, with a number of trays comprised within the range of from 150 to 200.

The cycle according to the present invention will be better understood by referred to the accompanying figures to which reference is made in the following examples, which are supplied solely for the purpose of better explaining the invention without limiting the scope of protection thereof.